The present invention relates to an improvement of a security apparatus for a power converter provided between a DC source and an AC system, by which an abnormal condition (e.g., power suspension) of the AC system is reliably detected so that the operation of the power converter is stopped.
Recent development regarding new energy is given to solar cells or fuel cells, in order that electric power thereof can be utilized practically. Although the form of energy obtained from these cells is DC power, it is convenient if the energy is converted into AC power. Meanwhile, as one of most efficient system utilizing the above energy, it is proposed that the power converter is connected to an AC system, thereby feeding overflow power of the converter to the AC system. (The reason why such overflow power feeding should be done is that output power of the converter is not always fully consumed.)
FIG. 1 shows a schematic circuit configuration of a solar cell power generator system by which DC energy from solar cell 11 is converted into AC power and the converted AC power can be sent to AC system 14. Solar cell or DC source 11 is connected to power converter 12 containing power converting circuit 13. By circuit 13, DC power is converted into AC power. The output of circuit 13 is connected to AC system 14, and system 14 is connected to load 15.
Usually, the generated energy of solar cell 11 is fully consumed by load 15. However, if the amount of the generated energy exceeds the consumed energy, or the generated energy overflows, the overflowed energy is fed to AC system 14. On the other hand, if the energy from solar cell 11 for load 15 lacks, the lacked part of the energy is filled up by energy from AC system 14.
Power converter 12, serving to achieve the above energy supply, contains phase detector circuit 21 which outputs sine wave signal S1. Signal S1 is synchronized with the AC voltage phase of AC system 14. Signal S1 is supplied to reference signal generator circuit 22.
Circuit 22 can be constructed as follows. DC voltage reference setter 221 provides DC voltage reference signal Vrefdc. DC voltage signal Vdc obtained from DC voltage detector circuit 222 is compared with signal Vrefdc. The result (Vrefdc-Vdc) of this comparison is amplified through DC voltage control circuit 223. Circuit 223 outputs AC current amplitude reference Irefac. Sine wave signal S1 is multiplied by reference Irefac in multiplier circuit 224. Then, circuit 224 outputs AC current reference signal S2.
Signal S2 obtained from circuit 22 is compared with AC current signal Iac obtained from current transformer 23. The result (S2-Iac) of this comparison is applied to switching control circuit 24. According to the above comparison result, circuit 24 sends on/off signal S3 to switching elements in converting circuit 13.
According to the above power converter 12, the DC voltage, input to power converting circuit 13, can be made constant. In addition, according to the operation of the minor loop (13-23-24) for a current control, a sine wave current, flowing into AC system 14 or into load 15, can be synchronized with the AC voltage phase, thereby achieving the power factor 1.
Incidentally, although not shown, when a control loop for AC output power is contained in power converter 12, signal S2 can be used as a reference for providing the maximum AC output power. This type of control is generally called as a maximum power control.
When AC system 14 of FIG. 1 is subjected to power suspension, abnormal voltage detector circuit 25, connected to AC system 14, is rendered active. Then, an abnormal state of the voltage is detected, and the operation of power converter 12 is safely stopped. The reason why such an abnormal state of the voltage can be detected is that since the generated AC power from converter 12 usually differs from the consumed power of load 15, excessively generated power renders the voltage of system 14 high, while insufficient power renders the voltage thereof low. However, if the generated power from converter 12 matches the consumed power of load 15, or if load 15 is fully powered by converter 12 only, since AC system 14 is free of the supply of power to load 15, conventional abnormal voltage detector circuit 25 can no longer detect the abnormal state of the voltage (power suspension) of AC system 14. This is the disadvantage of the prior art apparatus shown in FIG. 1.
If abnormal voltage detector circuit 25 cannot detect power suspension of AC system 14, even when a circuit breaker (not shown) is provided at AC system 14 and the breaker is cut off according to the occurrence of the power suspension, the voltage applied to load 15 is kept at high. Consequently, it is very dangerous when a maintenance staff gets AC system 14 in working order. This is also the disadvantage of the prior art apparatus shown in FIG. 1.